what is your shortest beneficial workout?

I can often fit in a lunchbreak ride during the week, I rode 2 days ago and yesterday evening. I would like to get out today, but due to my workload I am not going to get out for a real ride today. But I can probably get away for 15-20 min. I'm thinking of going for a short run, I have run long ago but nothing recently;
Maybe warmup for a couple mintues, a couple of minutes at a fast pace, maybe a couple sprints, cool down stretch... Would that be beneficial for xc race training? I should add I do regular 'runups' with my bike over my shoulder on one hill on my training route, but it's only about 30 seconds.

Personally I find running does a lot for my general fitness; but seems to have done nothing for bike fitness / speed. Subsequently I try to run only twice a week for about 30 minutes each session and mix up it up with a few sprints, etc...

Like yourself I am pretty "time poor" due to work / family / life commitments. However I find squeezing that extra cycling session or two on the trainer, or spin bike to be much more beneficial than running has been.

I don't generally follow James Wilson's recommendations for general training but for break from bike training i find his exercises to be pretty good. here is what I found working best in a time crunch :
1. Turkish get up (6 per side)
2. Single leg Dead Lift (about 20 per leg)
3. push up (15-25)
4. chin up (5-10)
5. Bulgarian squat (12-20)

repeat 3-5 times
of course, the sets will vary depending on your fitness and weight goals.

I would lean more towards the calisthenics side as well. Mainly because it's a PITA to get a bike loaded on car and ready to go, to get dressed in biking clothes, keeping your bike secure at work, etc. etc. etc. It's just a ton of extra wasted time on top of your precious exercise time.

If I want to maximize my time I would take a small gym bag with tennis shoes, shorts and t-shirt and do similar to rallymaniac.

To save prep time per Ponch, do Sheepo's workout but running instead of cycling. I think it will have more beneficial effect for cycling than calisthenics.

Other good option is 15-20 minutes is the perfect amount of time to get in a nice core workout. Most of us neglect it, but a strong core will make you a stronger cyclist and less prone to injury. Tommy D's Core Advantage has a bunch of cycling-specific core workouts that are about 15-20 minutes long.